1. Field of the Invention
This invention relates to a process for the production of pyridine, and, more particularly, to an improved process for making pyridine in high yield and selectivity during a prolonged period of operation.
2. Discussion of the Relevant Art
Pyridine is an important chemical in the manufacture of agricultural chemicals, e.g. herbicides and pesticides, and, of various pharmaceuticals. Pyridine also is useful as a solvent in the polymer and textile industries.
Pyridine is obtained either as a by-product of the coal tar industry or by chemical synthesis by a catalytic gaseous reaction between ammonia and a carbonyl compond. However, pyridine is found in only small amounts in coal tar, and, in chemical synthesis, the low yields and poor selectively, and short operation cycle, and lifetime, of the catalytic process is disadvantageous. The use of such catalysts in the chemical synthesis of pyridine is described, for example, in U.K. Patent Nos. 790,994 and 742,743; and in U.S. Pat. No. 3,272,825. These synthetic methods involve the condensation reactions of ammonia or amines with aldehydes or ketones.
Amorphous aluminosilicate catalysts provide a reasonable yield of pyridine at the beginning of the process; however, after repeated operation cycles some limitations appear which make these catalysts unacceptable from a commercial standpoint. Specifically, (1) large amounts of carbonaceous deposits form which reduce the activity of the catalyst to below acceptable levels; (2) the selectively of the reaction for the desired pyridine and alkylpyridine becomes very poor; (3) the catalysts become thermally unstable; and (4) the activity of the used catalyst is difficult to restore completely by regeneration.
Crystalline aluminosilicate catalysts having a silica to alumina ratio of 12 to about 1000 and a constraint index within the range of 1 to 12 have been used instead of amorphous aluminosilicates in such reactions. See, e.g. U.S. Pat. Nos. 3,728,408; 4,220,783 and 4,675,410. These crystalline catalysts have been provided in an attempt to improve the performance of amorphous catalysts. However, the yield of pyridine, and selectivity operation life, and restorability of the catalyst after regeneration still has not been satisfactory. Specifically, the yield of pyridine is low, the ratio of pyridine to picolines is not suitable, and the yield of the desired pyridine product decreases sharply after several reaction/regeneration cycles.